Some rough mutants of gram-negative bacteria cannot synthesize the entire polysaccharide moiety of their lipopolysaccharide. Their toxic site seems to be the lipid moiety (lipid A), and this is equally active in a number of endotoxicity assays. On the other hand, in some of the beneficial effects of endotoxins, such as enhancement of the non-specific resistance to infections or to tumors, great differences were found (Ng et al, J. Bact. 126:511, 1976). The objective of this project is to identify those relatively minor structural differences among the mutants which may be responsible for the presence or absence of the anti tumor resistance enhancing effect.